Activity of ceftolozane/tazobactam, imipenem/relebactam and ceftazidime/avibactam against clinical Gram-negative isolates—SMART United States 2019–21

Abstract Background Ongoing national and international surveillance efforts are critical components of antimicrobial stewardship, resistance monitoring, and drug development programs. In this report, we summarize the results of ceftolozane/tazobactam, imipenem/relebactam, ceftazidime/avibactam and comparator agent testing against 10 509 Enterobacterales and 2524 Pseudomonas aeruginosa collected by USA clinical laboratories in 2019–21 as part of the SMART global surveillance programme. Methods MICs were determined by CLSI broth microdilution and interpreted using 2023 CLSI M100 breakpoints. Results Most Enterobacterales were ceftazidime/avibactam susceptible (>99%), meropenem susceptible (99%) and ceftolozane/tazobactam susceptible (94%). Non-Morganellaceae Enterobacterales were also highly susceptible to imipenem/relebactam (99%). Ceftolozane/tazobactam inhibited 94% of Escherichia coli and 89% of Klebsiella pneumoniae with ceftriaxone non-susceptible/non-carbapenem-resistant phenotypes. Against P. aeruginosa, ceftolozane/tazobactam (97% susceptible) was more active than ceftazidime/avibactam (95%) and imipenem/relebactam (91%). MDR and difficult-to-treat resistance (DTR) phenotypes were identified in 13% and 7% of P. aeruginosa isolates, respectively. Ceftolozane/tazobactam remained active against 78% of MDR P. aeruginosa (13% and 23% higher than ceftazidime/avibactam and imipenem/relebactam, respectively) and against 74% of DTR P. aeruginosa (24% and 37% higher than ceftazidime/avibactam and imipenem/relebactam, respectively). Length of hospital stay at the time of specimen collection, ward type and infection type resulted in percent susceptible value differences of >5% across isolate demographic strata for some antimicrobial agent/pathogen combinations. Conclusions We conclude that in the USA, in 2019–21, carbapenem (meropenem) resistance remained uncommon in Enterobacterales and ceftolozane/tazobactam was more active than both ceftazidime/avibactam and imipenem/relebactam against P. aeruginosa.


Introduction
Ongoing national and international surveillance efforts that generate and publish current data are critical components of antimicrobial stewardship, resistance monitoring and drug development programmes.CLSI reference method testing and reporting of the in vitro activities of newer parenteral antimicrobial agents such as the β-lactam/β-lactamase inhibitor combinations ceftolozane/tazobactam, imipenem/relebactam and ceftazidime/avibactam is important because these agents may not be tested routinely in clinical laboratories given their use may be reserved for selected patients or they may not be included on the panels/cards used by laboratories for routine automated antimicrobial susceptibility testing.The absence of routine testing of newer agents risks prospective identification of emergent unanticipated anomalies in susceptibility.
Ceftolozane/tazobactam was approved by the US FDA in 2014, ceftazidime/avibactam in 2015 and imipenem/relebactam in 2019.2][3] IDSA currently lists imipenem/relebactam and ceftazidime/avibactam as preferred or alternative treatments (depending on infection source) for carbapenem-resistant Enterobacterales (CRE) infection. 4IDSA also recommends ceftolozane/tazobactam, imipenem/relebactam and ceftazidime/avibactam as preferred options for the treatment of serious infections (suspected or documented) caused by Pseudomonas aeruginosa with a difficult-to-treat resistance (DTR) phenotype. 4,5[9][10][11][12][13] The current report provides an update on antimicrobial susceptibility testing results for Enterobacterales and P. aeruginosa isolates submitted to the SMART (Study for Monitoring Antimicrobial Resistance Trends) global surveillance programme from 2019 to 2021 by participating medical centre laboratories in the USA.

Bacterial isolates
In 2019-21 the SMART global surveillance programme requested that each participating medical laboratory collect consecutive isolates of Gram-negative bacilli from bloodstream infections (50 isolates/year), intra-abdominal infections (50 isolates/year), lower respiratory tract infections (100 isolates/year) and urinary tract infections (50 isolates/ year).Only one isolate per patient per species per year was accepted.Isolates were collected without regard to specific species quotas.Twenty-four medical centre laboratories in 16 states in the USA collected 14 177 isolates of Gram-negative bacilli in 2019-21 as part of the SMART global surveillance programme (Table S1, available as Supplementary data at JAC-AMR Online).Sixteen of the 24 medical centre laboratories participated in all 3 years of isolate collection, four laboratories in 2 years, and four laboratories in 1 year.The isolates collected by the combined 24 medical centre laboratories in 2019-21 comprised 2937 bloodstream infection isolates (20.8% of all isolates), 2377 intra-abdominal infection isolates (16.8%), 5706 lower respiratory tract infection isolates (40.4%) and 3036 urinary tract infection isolates (21.5%); for 61 isolates (0.4% of all isolates) the infection source was not specified (Table S2).All isolates were sent to IHMA (Schaumburg, IL, USA) where organism identity was confirmed using MALDI-TOF MS (Bruker Daltonics, Billerica, MA, USA) and antimicrobial susceptibility testing was performed.

Antimicrobial susceptibility testing
MICs were determined using the CLSI broth microdilution method on inhouse prepared panels at IHMA and interpreted using 2023 CLSI breakpoints. 14,15Avibactam was obtained from Advanced ChemBlocks, Inc., (Burlingame, CA, USA), ceftolozane, imipenem and relebactam from Merck & Co., Inc., (Rahway, NJ, USA) and tazobactam from USP.Other antimicrobial agents were purchased from commercial sources.
A ceftriaxone non-susceptible/non-CRE phenotype was defined for Escherichia coli and Klebsiella pneumoniae isolates testing with a ceftriaxone MIC of ≥2 mg/L and an ertapenem MIC of ≤0.5 mg/L and was intended to approximate the subset of ESBL-producing, carbapenem-susceptible isolates amongst all isolates of those two species.MDR was defined as an isolate testing as resistant to ≥3 sentinel drugs: amikacin, aztreonam, cefepime, colistin, imipenem, levofloxacin and piperacillin/tazobactam. DTR was defined as an isolate testing as non-susceptible (intermediate or resistant) to both β-lactams (aztreonam, cefepime, ceftazidime, imipenem, meropenem, piperacillin/tazobactam) and fluoroquinolones (levofloxacin). 5The definition of DTR excluded antimicrobial susceptibility testing results for ceftolozane/tazobactam, imipenem/relebactam and ceftazidime/avibactam.
Morganellaceae (Morganella, Proteus, Providencia) were excluded from susceptibility analyses for imipenem/relebactam because the CLSI breakpoints for that agent do not apply to the family Morganellaceae. 14he CLSI M100 document also states that Morganellaceae are intrinsically less susceptible or resistant to imipenem (compared with other species of Enterobacterales) by a mechanism independent of β-lactamase production; 14,16 relebactam would not be expected to enhance imipenem's activity against isolates of Morganellaceae.
Statistical analyses of percent susceptible values across isolateassociated strata [e.g.hospital length of hospital stay (LOS), ward type, infection type] was not performed because of the large number of isolates tested where differences in percent susceptible values of <1% are statistically significant by χ 2 testing.
Meropenem percent non-susceptible values (i.e., CRE) were ≤2% for both Enterobacterales and NME across hospital LOS, ward type and infection type strata (Table 2).Percent susceptible values for ceftolozane/tazobactam, piperacillin/tazobactam, cefepime, ceftazidime and aztreonam were >5% higher for Enterobacterales isolates from patients with LOS <48 h than ≥48 h.Percent susceptible values for piperacillin/tazobactam were also ≥5% higher for isolates from patients hospitalized in non-ICU wards than in ICU wards.Percent susceptible values for ceftolozane/tazobactam, imipenem, piperacillin/tazobactam, aztreonam and levofloxacin varied by ≥5% across the four infection type isolates tested.Percent susceptible values were lower for lower respiratory tract isolates than for isolates from other infection types for all agents tested except levofloxacin.Against NME, imipenem/relebactam percent susceptible values varied by <2% for isolates stratified by hospital LOS, ward type and infection type.Across the four US census regions, only percent susceptible values for aztreonam against Enterobacterales differed by >5% (82.0% in the West compared with 87.1% in the Northeast and 87.2% in the Midwest (Table S5).
Ceftolozane/tazobactam and amikacin (both 97% susceptible) were the most active agents tested against P. aeruginosa, followed by ceftazidime/avibactam (95%) and imipenem/relebactam (91%); 79%-82% of isolates were susceptible to meropenem, piperacillin/tazobactam, cefepime and ceftazidime, while all other β-lactams and levofloxacin had percent susceptible values of 71% or less (Table 1).Amikacin was the most active agent (92% susceptible) against ceftazidime non-susceptible and meropenem non-susceptible isolate subsets of P. aeruginosa, followed by ceftolozane/tazobactam (83.0% and 88.7% susceptible, respectively).C/T, IMR and CZA activity against US GNB MDR and DTR phenotypes were identified in 13.4% and 6.6% of P. aeruginosa isolates, respectively (Table 1).The prevalence of MDR and DTR isolates of P. aeruginosa was higher among isolates collected from patients with hospital LOS ≥48 h than <48 h and MDR isolates were more prevalent among patients in ICU wards than non-ICU wards, while the DTR rate was similar (0.2% difference) in ICU and non-ICU wards (Figure 1).MDR and DTR phenotypes were most common among lower respiratory tract infection isolates (15.7% and 7.6%, respectively) and least common among bloodstream infection isolates (4.9% and 4.0%).
Among P. aeruginosa, percent susceptible values for piperacillin/tazobactam, ceftazidime and aztreonam were >5% higher for isolates from patients with hospital LOS <48 h than ≥48 h (Table 2).Percent susceptible values for all agents differed by <5% between isolates from patients hospitalized in non-ICU wards and in ICU wards.Percent susceptible values for all agents except ceftolozane/tazobactam, ceftazidime/avibactam and amikacin varied by >5% across the four infection type isolates tested.Percent susceptible values were lower for lower respiratory tract isolates than isolates from other infection types for all agents tested except imipenem/relebactam (lowest for intra-abdominal infection isolates).Across the four US census regions, percent susceptible values for imipenem/relebactam, imipenem, meropenem, piperacillin/tazobactam, cefepime, ceftazidime, aztreonam and levofloxacin were >5% lower in the Northeast region than at least one other region (Table S5).

Discussion
In the USA in 2019-21, meropenem resistance remained uncommon (1%) among clinical isolates of Enterobacterales (Table 1), including across LOS (i.e.community-acquired and hospital-acquired infection), ward type (ICU and non-ICU wards) and infection type (bloodstream, intra-abdominal, lower respiratory tract and urinary tract) strata (Table 2), confirming data published by other investigators surveying Enterobacterales isolates from medical centre laboratories across the USA in 2018-20 17,18 and earlier. 19The majority of clinical isolates of meropenem non-susceptible Enterobacterales in the USA carry a KPC enzyme. 19 The higher percent susceptible value for ceftolozane/tazobactam (94.2%) compared with piperacillin/tazobactam (87.8%) against Enterobacterales isolates may reflect the presence of the OXA-1 penicillinase in some ESBL non-CRE isolates. 20n contrast to Enterobacterales, recent (2019-21) meropenem (79%) and imipenem (65%) percent susceptible values for US clinical isolates of P. aeruginosa (Table 1) preclude carbapenem use as empirical therapy when this pathogen is highly suspected.Cephalosporins, piperacillin/tazobactam, aztreonam and levofloxacin also tested with percent susceptible values ≤82% (Table 1).2][23][24][25] Previous studies have also reported that imipenem/relebactam demonstrated greater activity than ceftazidime/avibactam against ceftolozane/tazobactam non-susceptible clinical isolates of P. aeruginosa collected in the USA, 13,18 suggesting that both ceftolozane/tazobactam and imipenem/relebactam be considered for routine antimicrobial susceptibility testing of clinical isolates of P. aeruginosa.Our study (Table S6) confirmed this observation with more recent data.We also observed that ceftazidime/avibactam was more active than imipenem/relebactam against meropenem non-susceptible isolates of both P. aeruginosa and NME (Enterobacterales), which is not unexpected given the differences in cellular uptake of the two β-lactam constituents and differences in mechanisms of resistance for these two agents (Table 1). 7,8,13,26n the USA almost all (99%) of carbapenem, imipenem/relebactam or ceftolozane/tazobactam non-susceptible isolates of P. aeruginosa do not carry an acquired β-lactamase, including carbapenemases, 13,27 in contrast to many regions in the world where carbapenemase rates among resistant P. aeruginosa isolates are higher. 28,29Lower ceftolozane/tazobactam, imipenem/relebactam and ceftazidime/avibactam susceptibilities have been reported in regions outside the USA in association with higher numbers of isolates carrying MBLs. 30solates of P. aeruginosa in the current study were also highly susceptible to amikacin (97.0%); however, aminoglycoside use is associated with well-established toxicities and therapeutic limitations, and the use of this agent class is strongly discouraged.Both the CLSI 14 and EUCAST 31,32 have recently introduced susceptibility testing and reporting restrictions for aminoglycosides to further dissuade their use.
0][11]13 In the current study, 78% of MDR P. aeruginosa were ceftolozane/tazobactam susceptible compared with 66% ceftazidime/avibactam susceptible and 56% imipenem/relebactam susceptible.In the current study, 7% of P. aeruginosa isolates had a DTR phenotype; percent susceptible rates for DTR isolates exceeded 50% for only amikacin (84% susceptible) and ceftolozane/tazobactam (74% susceptible).Gram-negative bacilli with a DTR phenotype are associated with therapeutic failure and mortality, particularly in severely ill patients. 5,6The significant numbers of both MDR and DTR P. aeruginosa, as well as emerging resistance and MDR in all human pathogens, underscores the necessity for strong and consistent antimicrobial stewardship programmes across all patient types, with practices that are independent of hospital LOS, ward type or infection type.
The strengths of the current study are that it collected isolates from a relatively consistent set of medical centre laboratories in the USA for a recent 3 year period according to a consistent protocol that did not include species quotas and used reference broth microdilution antimicrobial susceptibility testing performed in a central laboratory to test three newer β-lactam/β-lactamase inhibitor combinations and eight established agents.Study limitations include that there was some change in study participation by individual medical centre laboratories over the 3 years of the study and that it used quotas to collect isolates from different infection types that may affect the overall estimates of resistance and β-lactamase prevalence.The data presented in this study may be limited by the small annual sample size (250 Gram-negative isolates per medical centre per year).The data generated from isolates submitted by participating medical centres within the USA should not be extrapolated to represent all isolates or geographical areas within the country.

Table 1 .
Antimicrobial susceptibility testing results for isolates of Enterobacterales, NME and P. aeruginosa, including resistant isolate subsets, collected by the SMART global surveillance programme in the USA from 2019 to 2021 C/T, ceftolozane/tazobactam; IMI/REL, imipenem/relebactam; CZA, ceftazidime/avibactam; IMI, imipenem; MEM, meropenem; P/T, piperacillin/tazobactam; FEP, cefepime; CAZ, ceftazidime; ATM, aztreonam; LVX, levofloxacin; AMK, amikacin; NME, non-Morganellaceae Enterobacterales; CRO, ceftriaxone; CRE, carbapenem-resistant Enterobacterales; DTR, difficult-to-treat resistance; NA, not available; NS, non-susceptible.a Too few isolates of E. coli (n = 10) and K. pneumoniae (n = 41) with a meropenem-non-susceptible phenotype were identified to be included in the table.b Susceptibility to levofloxacin was not available for Salmonella spp.(n = 17) because the tested concentration range did not extend low enough for the Salmonella-specific CLSI susceptible breakpoint for that agent.c Susceptibility to amikacin was not determinable for Enterobacterales because the tested concentration range did not extend low enough to include the revised 2023 CLSI susceptible breakpoint for that agent.

Table 2 .
Antimicrobial susceptibility testing results for isolates of Enterobacterales, NME, and P. aeruginosa collected by the SMART global surveillance program in the United States from 2019 to 2021 stratified by length of hospital stay at the time of specimen collection, ward type, and infection type Numbers of isolates in individual strata may not total to the numbers of isolates reported in Table1because some isolates do not have a specified LOS, patient location or infection type and some isolates were from emergency departments.b Susceptibility to levofloxacin was not available for Salmonella spp.(n = 17) because the tested concentration range did not extend low enough for the Salmonella-specific CLSI susceptible breakpoint for that agent.Susceptibility to amikacin was not determinable for Enterobacterales because the tested concentration range did not extend low enough to include the revised 2023 CLSI susceptible breakpoint for that agent.